This invention relates to ultrasonic imaging and more particularly to a method of obtaining volume backscatter images.
The measurement of intrinsic acoustic parameters of tissue, such as absorption coefficient, sound velocity, and scattering coefficient, holds great promise for ultrasonic imaging. Moreover, spatially localized measurements of an intrinsic tissue parameter may permit tissue characterization using ultrasound. The results presented in a number of studies published over the last few years indicate that the ultrasonic backscatter coefficient is a sensitive index of tissue pathologies. These studies have shown that quantitative measurements of the backscatter coefficient can be used to identify specific tissue types. This invention concerns a specific method of using backscatter measurements for quantitative imaging.
Conventional B-scans are crude approximations to a backscatter image. The B-scan is a map of the signal intensity returning to the transducer from every point in the image. The volume backscatter coefficient, however, is a measure of the backscatter efficiency per unit volume. Because many factors can affect the absolute signal intensity, the B-scan is not an image of the scattering efficiency. To obtain a backscatter image, all factors influencing the absolute signal intensity must be taken into account.
A number of problems need to be solved which are not present in controlled laboratory studies in order to get quantitative volume backscatter images in the clinic. The principal problems can be segregated into two classes: detector artifacts and progagation effects. Detector artifacts arise primarily because conventional B-scan systems use piezoelectric transducers as both the transmitter of the interrogating ultrasonic pulse and the receiver of the backscattered pulse. Due to the phase sensitive nature of piezoelectric transducers, the detection of waves propagating through inhomogeneous media can be compromised by phase cancellation effects; phase distortions resulting from propagation through inhomogeneous media can dramatically alter the character of B-scan images. It has been shown by the inventor that these distortions do not significantly change estimates of the volume backscatter coefficient over a volume of tissue.